JP2525884B2 - Decontamination method for radioactive solid waste - Google Patents

Description

TECHNICAL FIELD The present invention relates to a decontamination method suitable for improving the decontamination efficiency of radioactive solid waste.

[Prior Art] The basic policy is to reduce the amount of waste generated from nuclear power plants and to reduce the volume of the generated waste. In line with this policy, liquid waste is being significantly reduced in volume. On the other hand, compression volume reduction and melting volume reduction have been proposed for metal waste, but it is difficult to secure a considerably large volume reduction.

[Problems to be Solved by the Invention] Meanwhile, radioactive solid waste such as radioactive metal waste may be converted into general waste by decontamination. This is because radioactive metal waste is generally only surface contamination and surface contamination can be removed.

Here, the technical problem is how to efficiently find the contaminated portion on the surface of the metal waste and how to efficiently remove the pollutant. This is because the waste is not uniformly contaminated and it is inefficient to decontaminate it many times, and if the waste cannot be removed even at one surface, the entire waste will be decontaminated. Because it will not be possible. Such a problem occurs, for example, when the surface of the metal waste is not uniform and particularly has uneven portions such as welded portions.

On the other hand, if the contaminated portion can be selectively removed according to the contamination status of the waste, the decontamination efficiency can be significantly improved. This is because the optimum decontamination method can be selected according to the surface condition of the contaminated site and the number of times of decontamination can be reduced.

In decontamination, it is important to confirm whether the waste has been completely decontaminated, but the boundary value of the radioactivity concentration between radioactive waste and general waste is extremely low, and the There is a problem that it takes a long time to measure radioactivity. It is necessary to measure the radioactivity of all decontaminated waste to confirm the effect of decontamination (whether it can be treated as non-radioactive waste). It is desirable to measure the active concentration.

An object of the present invention is to improve the decontamination efficiency by decontaminating a contaminated part by selecting an optimum decontamination method for the contaminated part of the radioactive solid waste.

[Means for Solving the Problems] The above object can be achieved by the configurations described in each of the claims.

[Operation] As described in claims 1 to 3, precise measurement for measuring the radioactivity concentration of each section of the surface of the radioactive solid waste is performed to identify the radioactive contamination site and decontaminate the site. As a result, decontamination can be performed efficiently.

[Examples] Examples of the present invention will be described below.

First Embodiment A first embodiment of the present invention will be described with reference to the flowchart shown in FIG.

The wastes to be decontaminated are radioactive wastes such as metal wastes, rubbers, plastics, and trees whose surfaces are contaminated only. If the surface contaminations are removed, they may be treated as ordinary wastes. There is something. Waste that contains radioactive material inside and is contaminated inside is not subject to pollution.

The form in which radioactive material adheres to the surface of waste depends on the environment in which it is used, and it is difficult to identify the form. Generally, it is divided into those that are softly attached and those that are firmly fixed.

Moreover, the degree of difficulty of decontamination varies depending on the properties of the waste surface. If the surface is smooth, it is generally easy to decontaminate, but if there is unevenness on the surface, it is difficult to remove radioactive substances that have entered into narrow gaps, and decontamination cannot be done easily, and contaminants are removed together with the base material. There is a need. Thus, the suitable decontamination method differs depending on the pollution status of the waste.

Moreover, the measurement of the radioactivity concentration (the amount of radioactivity per unit surface area) is carried out by detecting γ rays. For this reason,
In order to improve the measurement sensitivity, it is necessary to efficiently capture γ rays. If the surface area to be measured is wide, assuming that the radioactivity distribution is uniform, the number of γ rays to be measured increases. On the other hand, when dividing and measuring the surface area to be measured, the number of γ-rays that can be captured decreases, and the measurement sensitivity, that is, the lower limit of measurement cannot be reduced. , Which increases the measurement time.

In general, when the measurement surface range is limited, a diaphragm device 2 called a collimator is attached to the radiation detector 3 for measurement as shown in FIG. This limits the range of gamma rays entering the detector 3. When measuring the radioactivity concentration in each division by dividing the measurement surface range in this way, when trying to obtain the same sensitivity and the same detection limit as when measuring the whole at once, it is proportional to the number of divisions. Since this increases the measurement time for each category, it is not a good idea to make such a measurement for all wastes.

Since it is necessary to measure all wastes whether or not the wastes are radioactive wastes, first measure the radioactivity of all the wastes at once to shorten the measurement time. Try. If the radioactivity concentration measured here is below a predetermined value, the waste may be treated in the same way as general industrial waste. However, if the radioactivity concentration measured here is equal to or higher than a predetermined value, the waste will be treated as radioactive waste. The measurement for this segment is the primary measurement. Waste that is determined to be radioactive waste in the primary measurement is subject to decontamination.

Next, the applicable decontamination method is selected according to the properties of this waste and the state of contamination. Here, whether the pollution is due to oil,
The state of contamination is classified according to whether it is fixed contamination or whether the radioactivity concentration is high or low. This is because the effective decontamination method differs depending on the contamination situation.

For example, when radioactive substances such as oils are attached, it is effective to wash them with freon and an organic solvent (kerosene, alcohol, carbon tetrachloride, etc.). However, cleaning with these agents is not very effective for adhering contaminants. In addition, with these decontamination methods by washing, it is difficult to remove even the contaminants that have entered the minute voids on the surface of the waste, and the limit is to reduce the radioactivity concentration to about 1/10. In order to obtain the dyeing effect, it is necessary to combine with other decontamination methods.

In the case of contamination due to adherence, it is necessary to select a decontamination method that can remove adhered matter. As such a decontamination method,
There are physical and chemical effects. The decontamination method by physical action is to remove the waste surface layer with a brush, heat, etc., and water jet (10-100 kg /
cleaning with high-pressure water in cm ^2), the surface layer by plasma arc scarfing, a combination of brush cleaning and water knife, and the like surface switching deleting method by byte. However, even with these decontamination methods, it is difficult to completely remove even the contaminants that have entered the minute voids on the surface of the waste, and there is the possibility that the contaminants on the surface layer of the waste will be re-contaminated. Is limited to about 1/10, and it is necessary to combine with other decontamination methods to obtain more decontamination effects.

In addition, as the decontamination method by chemical action, chemical decontamination in which the waste surface layer is chemically dissolved using an acidic chemical solution,
There is an electrolysis dyeing method in which the surface layer of waste is dissolved by using an electrochemical action. This method can prevent recontamination from the removed material by controlling the water quality and radioactivity concentration of the decontamination solution to remove the waste surface layer, and reduce the radioactivity concentration of the waste to about 1/100. it can.

The features of typical decontamination methods have been described above, but it is necessary to select the decontamination method in accordance with the features of these decontamination methods and the characteristics of waste. For this reason, it is necessary to accurately understand the characteristics of waste, especially the characteristics related to radioactivity concentration.

The waste whose radioactive concentration is measured by the primary measurement and whose result is less than the specified value may be treated as general waste. On the other hand, for those judged to be radioactive waste as a result of the first measurement, the applicable decontamination method is selected based on the radioactive concentration of the waste and information on the material, shape, etc. At this time, the shape and surface condition of the waste are not uniform, and even in the same metal waste, there are smooth surfaces and weld beads, but in terms of classification, it is generally determined by the dominant part of the waste. Target. That is, even if there is a weld bead portion, if the smooth portion has a large area, the waste is classified as a waste having a smooth surface.

Then, decontamination is performed by the decontamination method selected above. For the waste that has been decontaminated, as the second measurement, the radioactive concentration of the entire waste is measured again to confirm the decontamination effect, and the waste whose radioactive concentration is confirmed to be below the specified value May be treated as municipal waste.

Waste that has not been completely decontaminated by the second measurement will be decontaminated again, but it is inefficient to repeat the same decontamination many times, so decontamination is efficient. In order to carry out the above, before decontaminating the waste after decontamination again, the characteristics are confirmed and the optimum decontamination method is selected. For this purpose, the contamination distribution on the waste surface is measured by precise measurement. Specifically, as shown in FIG. 2, the detection range of the radiation detector 3 is limited by the diaphragm device 2 such as a collimator,
This is positioned with respect to each section of the waste surface, and the contamination distribution is measured by measuring the radioactivity concentration of each surface section of the waste, and the information is output to the CRT or printer. The detector consists of a detector driver that moves the detector to each part of the waste, a diaphragm device that defines the detection range (collimator, etc.), an analyzer that analyzes the output waveform of the detector (multichannel analyzer, etc.), and data. It consists of a data processing device that analyzes and outputs to a CRT, etc. A NaI or semiconductor detector is used as the detector.

Based on the data obtained in this way,
This time, the applicable decontamination method will be selected based on the characteristics of the residual contamination points of the waste, and this will be applied to the residual contamination points. For example, if the residual contamination portion is the weld bead portion, a decontamination method suitable for decontaminating the weld bead portion is selected, and this is performed only on the weld bead portion. As a result, the optimum decontamination method can be carried out only on the contaminated part, the operation efficiency of the treatment equipment can be improved, and the ratio of the waste that can be decontaminated to be general waste can be improved.

Those that cannot be decontaminated even if the above-mentioned decontamination work is repeated N times (N> 1) are provided for disposal as radioactive waste.

In the first measurement for confirming the radioactivity concentration before decontamination, it is sufficient if it is possible to determine whether the entire waste is the target and whether it is contaminated with radioactive substances as a function.
It is not rational to apply precision measurements that are time consuming. Also, it is sufficient if the second measurement, which is a confirmation after decontamination, can judge whether the waste is contaminated with radioactive substances and whether the waste as a whole is ON / OFF. Precision measurement cannot be completely decontaminated by decontamination. Good to apply to waste.

Example 2 When the amount of waste is small, as shown in FIG.
The same measurement device can also be used for the secondary measurement, secondary measurement and precision measurement. In this case, it is necessary to use a device capable of measuring the contamination distribution (for example, a device as shown in FIG. 2) as the measuring device.

Example 3 Depending on the type and situation of waste, as a modified example of Example 1, the waste determined to be radioactive in the primary measurement is immediately subjected to precise measurement, and the decontamination method selected based on it is carried out. However, after that, it is possible to go through the secondary measurement and the subsequent process in the first embodiment.

Example 4 Alternatively, as a modified example of Example 1, the decontamination method selected based on the result of the primary measurement was carried out, and then the precise decontamination was immediately carried out and the decontamination method was carried out again based on the result. It is also possible to select and carry out the decontamination method, and then carry out the secondary measurement in Example 1 and the subsequent process.

Application example When measuring the radioactivity concentration of waste, it is important to lower the lower limit of measurement (detection limit). This is because the boundary value of the radioactivity concentration that can be regarded as radioactive waste and general waste is extremely small. Further, since such measurement of the radioactivity concentration is performed in the controlled area, it is necessary to shield the surroundings sufficiently in order to lower the lower limit of measurement so as not to be affected by the surrounding radiation. In this way, a radioactivity concentration measuring device with a small detection limit and good performance will be a device with heavy equipment such as shielding, and it will take a long time for measurement, and from the viewpoint of preventing contamination inside the measuring device during operation, it will be extremely It is desirable to avoid measuring waste with high radioactivity. Further, considering the distribution of the radioactive concentration of waste, it is not always necessary to have a high-performance radioactive concentration measuring device for all radioactive wastes.

Therefore, in the fifth embodiment, in measuring the radioactive concentration of waste, rough measurement is first performed, and measurement is sequentially performed using a measuring instrument with good detection sensitivity, thereby reducing the number of measuring devices and operating efficiency. To improve.

The fifth embodiment will be described with reference to FIGS. 4 and 5. Since there are many kinds of radioactive waste, the radioactive concentration of radioactive waste usually has a certain distribution when statistics are taken for many wastes. Now, assume that this distribution is a distribution as shown in FIG. A large number of wastes having the radioactivity concentration distribution shown in Fig. 5 are measured by using three kinds of measuring instruments of A, B and C having different detection limit values as shown in Table 1, respectively.

(Note) a <b <cc <(Boundary value concentration that can be regarded as general waste) As shown in the flow chart in FIG. 4, first, using measuring instrument A as the primary measurement. All wastes are measured, and the wastes with detected radioactivity are judged as radioactive wastes. The waste determined by the measuring instrument A to be below the detection limit value (that is, the waste whose radioactivity is not detected by the measuring instrument A) is measured by the measuring instrument B having the next smallest detection limit value. The waste in which the radioactivity is detected by the measurement with the measuring device B is determined to be radioactive waste. The waste determined by the measuring instrument B to be below the detection limit value (that is, the waste whose radioactivity was not detected by the measuring instrument B) is
Next, the measurement is performed by the measuring device C having a low detection limit value. Wastes whose radioactivity was detected by the measuring instrument C are judged to be radioactive wastes, and those not detected are judged to be general wastes. here,
The detection limit value c of the measuring device C needs to be smaller than the radioactivity concentration that is the boundary value with general waste. Otherwise, the waste cannot be certified as non-radioactive.

As for the processing capacity of the device, the measuring instrument A has 100% capacity of the waste to be processed (capacity for examining all waste).
Is required, the measuring instrument B needs 20% capacity, and the measuring instrument C needs 5% capacity. Then. For example
When treating 100 solid waste, measuring instrument A 100 solid x 2 minutes = 200 minutes measuring instrument B 20 solid x 10 minutes = 200 minutes measuring instrument C 5 solid x 40 minutes = 200 minutes It is possible and the total measurement work is 20
It can be completed in 0 minutes.

On the other hand, if all the wastes are to be treated by the measuring instrument C, the measuring instrument C 100 times x 40 minutes = 4000 minutes, which is 20 times longer than the above method. Further, in order to perform processing in the same time as above, 20 expensive measuring instruments C having high measurement sensitivity are required.

On the other hand, in the present embodiment, one expensive high-precision measuring device C and two measuring devices A and B having a lower precision than the above, for a total of three, are equivalent to 20 expensive measuring devices C. Can do the job.

The above example is based on the assumption of the radioactive concentration of waste as shown in Fig. 5. The radioactive concentration of radioactive waste has a certain distribution, and this distribution may change depending on the target. Specifically, the same effect as described above can be obtained by the same method as described above.

Thus, in measuring the radioactive concentration of radioactive waste, a measuring device with high measurement sensitivity (that is, low measurement limit value) and a measuring device with low measurement sensitivity but high processing speed (that is, short measurement time) are used. By measuring wastes in combination, it is possible to measure efficiently and inexpensively. At the same time, the waste can be classified according to the radioactivity concentration.

Although the case where three types of measuring devices are used has been described above, such an effect can be obtained even when two or more types of measuring devices having different measurement limit values are used.

Such a measuring method is applied to the primary measurement and the secondary measurement in the first embodiment. By doing so, it is possible to efficiently measure the radioactivity concentration at the time of decontaminating waste, improve the operation efficiency of the decontamination system, and reduce the number of measuring devices.

[Effects of the Invention] In radioactive decontamination of radioactive solid waste, the following effects are obtained by carrying out the decontamination method selected by measuring the contamination distribution by measuring the contamination of each section on the surface of the waste. .

The contaminated part of the waste can be identified, and the optimum decontamination method can be selected and implemented accordingly.

By applying the optimum decontamination method, it is possible to reduce the number of times of decontamination repetition and improve the decontamination efficiency.

[Brief description of drawings]

FIG. 1 is a flow chart of decontamination treatment of radioactive waste according to an embodiment of the present invention, FIG. 2 is a schematic diagram of a radioactive contamination distribution measuring device, and FIG. 3 is a radioactive waste according to another embodiment of the present invention. Decontamination process flow chart, Fig. 4 is a measurement flow chart for measuring the radioactive concentration of waste using measuring devices with different detection limits, and Fig. 5 is the statistical distribution frequency of the radioactive concentration of radioactive waste. It is an illustration figure of.

Claims (3)

(57) [Claims] 1. A radioactive solid waste for which decontamination is performed on the radioactive solid waste by performing precise measurement to measure the radioactive concentration of each section on the surface of the radioactive solid waste to identify the radioactive contamination site. In the decontamination method, the primary measurement for determining whether or not the solid waste is a radioactive solid waste by measuring the radioactivity concentration of the entire solid waste before performing the precise measurement. Decontamination of radioactive solid waste, which is characterized by performing the above-mentioned precise measurement on the waste judged to be radioactive solid waste as a result, identifying the radioactively contaminated site, and decontaminating the site. Method. 2. A radioactive solid waste for which decontamination is carried out by performing a precise measurement for measuring the radioactive concentration of each section of the surface of the radioactive solid waste to identify the radioactively contaminated site and carrying out decontamination on the site. In the decontamination method, the primary measurement for determining whether or not the solid waste is a radioactive solid waste by measuring the radioactivity concentration of the entire solid waste before performing the precise measurement. After performing the overall decontamination of the waste judged to be radioactive solid waste as a result, perform a precise measurement on the waste to identify the radioactive contamination site and decontaminate the site. A method for decontaminating radioactive solid waste, comprising: 3. A radioactive solid waste for which radioactive contamination is determined by performing precise measurement for measuring the radioactive concentration of each section on the surface of the radioactive solid waste to decontaminate the site. In the decontamination method, the primary measurement for determining whether or not the solid waste is a radioactive solid waste by measuring the radioactivity concentration of the entire solid waste before performing the precise measurement. After carrying out overall decontamination of the wastes judged to be radioactive solid wastes as a result, the solid wastes are still radioactive solid wastes by measuring the radioactivity concentration of the whole solid wastes. A secondary measurement is performed to determine whether there is any, and the precision measurement is performed on the waste determined to be radioactive solid waste as a result of this secondary measurement to identify the radioactive contamination site and Remove A method for decontaminating radioactive solid waste, which comprises performing dyeing.